NOVEMBEE 23, 1895.] 



SCIENCE. 



705 



calculations will be made by Mr. Davis. The 

 paper was discussed by Prof. Rees. 



Prof. Pupin then explained a method of 

 measuring alternating currents with a galva- 

 nometer. It consists in placing in the circuit 

 a primary cell and an electrolite cell whose 

 counter electro-motor is slightly greater than 

 that of the primary cell. Under these condi- 

 tions only one-half of the alternations passed 

 through the circuit, the other half being stopped 

 by the two cells. Experiments have shown 

 the availability of this method up to 600 alter- 

 nations per minute. 



J. F. Kemp, Secretary. 



GEOLOGICAL CONFEEENCE OF HARVARD UNI- 

 VERSITY, OCTOBER 22, 1S95. 



The Development of Oligoporus. By Robert T. 



Jackson. 



The following is an abstract of the results of 

 recent studies of the Palseoechinoidea. In Oli- 

 goporus the interambulacra terminate veutrally 

 in two plates, which present on their oral faces 

 a reentrant angle for the reception of a single 

 initial plate of the area. Proceeding dorsally, 

 new plates and uew columns of plates are 

 added, accenting by their appearance stages in 

 growth, as he had previously shown in Melon- 

 ites, until the full complement of the species is 

 attained. The single initial interambulacral 

 plate of Oligoporus was compared with a simi- 

 lar plate in Melonites, Lepidechinus, young 

 modern Cidaris, etc. At the ventral or younger 

 portion of the corona of Oligoporus there are 

 only two columns of ambulacral plates. The four 

 columns characteristic of the adult are derived 

 from these two by a drawing-out process. The 

 four columns of ambulacral plates of adult Oli- 

 goporus are the equivalent of the two outer and 

 two median columns of Melonites. These four 

 columns in both genera are the morphological 

 equivalent of the two columns seen in the am- 

 bulacra of Bothriocidaris, Cidaris, etc. 



Oligoporus, as shown by the development of 

 both ambulacral and interambulacral areas, is a 

 genus intermediate between Palteechinus and 

 Melonites. During the development of Oligo- 

 porus it passes through a Rhoechinus stage, and 

 later a Palseechinus stage. Melonites in its de- 

 velopment passes through an Oligoporus stage. 



An early stage in developing Echiuoderms 

 was named the ' protechinus ' stage. At this 

 stage are lirst acquired those features which 

 characterize the developing animal as a member 

 of the Echinoidea. The protechinus stage in 

 Echiuoderms is directly comparable to the pro- 

 toconch of Cephalous Mollusca, the protegulum 

 of Brachiopods, the protaspis of Trilobites, etc. 

 The Echinoderm at this period in its growth has 

 a single interambulacral plate (representing a 

 single column of such plates), and two columns 

 of ambulacral plates in each of the five areas. 

 This stage is seen in Oligoporus, Lepidechinus, 

 Goniocidaris and other genera ; it finds its rep- 

 esentative in au adult ancestral form, in the 

 primitive, oldest known genus of the class, 

 Bothriocidaris, of the Lower Silurian, which 

 has but one column of interambulacral and two 

 columns of ambulacral plates in each area. 



Species of Oligoporus and Melonites with few 

 intei-ambulacral columns are considered the 

 more primitive types, as they are represented 

 by stages in the development of those species 

 which acquire a higher number of columns iu 

 the adult. 



The structure of the ventral border of the 

 corona of Archseocidaris was described. It 

 presents a row of jslates partially resorbed by 

 the encroachment of the peristome, as iu modern 

 Cidaris, etc. Ambulacral and interambulacral 

 plates on the peristome were described in 

 Archseocidaris, also teeth and secondary spines 

 on the interambulacral plates of the corona. 



This paper contains a classification of Palseo- 

 zoic Echini based on the structvire and develop- 

 ment of the ambulacral and interambulacral 

 areas and the peristome. It will be published 

 in the Bulletin of the Geological Society of 

 America. 



Tidal Sand-cusjJs. F. P. Gulliver. 



In the rias, or drowned valleys, of the Puget 

 sound region, Washington, occur many cuspate 

 deposits of sand projecting from the valley sides 

 into the tidal inlets. West point, north of 

 Seattle, is the typical example (Coast Survey, 

 653 ; Geological Survey, Seattle sheet). These 

 points always project at right angles both to the 

 shoreline and to the general direction of in and 

 out flowing tidal currents. They vary in stage 



